The present invention relates to the printing arts. It finds particular application in conjunction with halftoners and hyperacuity printers. However, it is to be appreciated that the present invention will also find application in conjunction with other printing systems and applications in which accurate rendering of marks are advantageous.
According to human visual research, the frequency response or resolution of a printing system need only exceed the resolving power of the human visual system. Such resolving power is known as visual acuity. However, there are human visual considerations that require the placement of edges 10-60 times more accurately than that indicated by frequency resolution considerations. These requirements are based on hyperacuity, or the visual systems ability to differentiate locally misaligned edges to a much greater extent than the interreceptor spacing of the eye. In this case, it is not the frequency response (resolution) of the visual system that is most important, but the ability to reckon edges with high precision. Therefore, there was a need to be able to place edges or transitions in images in both fastscan and process (slow scan) directions with precision greater than that of the actual printer resolution. A detailed description of the relationship between the human visual system and printer resolution can be found in the Journal of Electronic Imaging, April, 1993, Vol. 2(2), pgs. 138-146, entitled "Hyperacuity Laser Imager" by Douglas N. Curry, which is hereby incorporated by reference.
High quality printing depends on many factors including how much fidelity information can be passed to the printer from a data source or image generator. A purpose of printer electronics is to convert high fidelity, sampled image representation into a high band width, high resolution bit stream for delivery to a photoreceptor. The image generator can be either binary or gray, and is not restricted to a specific resolution or gray (multi-bit) depth. In general, the gray depth over an image can be variable to accommodate a physical interface with the imager subsystem or to adjust the fidelity of image information sent to the printer. The image generator can also download in advance of printing, halftoning and thresholding information tuned to any special characteristics of the imager and the desired imaging response. This download file is not image data, but rather is the contents of a look-up table used for halftoning or thresholding.
A resampling interpolator is typically found in the electronic pathway between the image generators and the output. The interpolator electronically maps data spatial positions into arbitrary misalignment of the imager and its mechanical process, as well as its customary use which provides resolution conversion. It executes a standard image processing function of interpolation, which can be linear interpolation. For a hyperacuity printer, the spot on the photoreceptor is tracked by appropriate electronics taking into account electronic registration requirements. To obtain a resample, the spot's current position is measured in units of addressability in both x (fastscan) and y (slow scan or process) dimensions. The addressability is not necessarily coincident with one of the integer sample positions, but instead is fractionally positioned.
Halftoners are used to convert the resampled information from the resampling interpolator into a binary map when pictorials are to be rendered. The halftoner includes a halftoner memory which stores pregenerated halftone cells which represent intensity information (for example, 256 values), as well as integer screen positions for each cell. For example, a halftone cell may be formed of 8.times.8 values. The intensity and screen position values are used to address the halftoner memory and a multi-bit output is retrieved which drives a gray modulator.
Automoire occurs when an individual halftone cell is produced with systematic quantization error. Rational tangent techniques for accessing halftone memory exhibit no moire because they have zero quantization error. When generating halftones by this method, each new memory location is chosen as an offset from the last location, with the offset being an integer. Unfortunately, because the offsets are restricted to integers, the number of halftone frequencies and angles used for incrementing the screen positions are severely limited. However, if fractional offsets are used, moire is introduced into the image caused by the periodic and systematic accumulation of quantization error. Typically, the screen position generated by the accumulators is an irrational number which has a fractional component. The halftoner memory, however, does not use fractional address locations. Thus, the fractional component of the screen position is discarded, and only the integer values are used to address the memory. Thus, inaccurate marks are rendered.
In accordance with the present invention, a hyperacuity halftoner is provided which corrects for automoire phase error by using the fractional component of screen positions.